As we noted when the latest Airport Extreme Base Station firmware shipped, Time Machine now seems to recognize USB hard drives connected to the AEBS as valid backup locations. Was this feature added deliberately? Well, Glenn Fleishman over at TidBITS took the trouble to ask Apple about this and he reports that they told him that this is an unsupported feature (and not much else). This is unsurprising given that Apple never made mention of the feature connected with the firmware update (though it was originally promised before Leopard shipped). Fleishman himself even speculates that it was turned on by accident.
So what's the upshot? Apple is offering no support for using your AEBS this way, and so if you have a problem you're pretty much out of luck as far as they're concerned. Further, considering that there have been reports that the Airport Disk can be unreliable, it's probably not a good idea to depend on an Airport Disk and Time Machine for your only backup.